Optical pickup device including a hologram lens section with an oval diffraction grating pattern

ABSTRACT

A two-focal-point pickup device capable of suppressing the generation of glitch or bump in reproduced signal. The optical pickup device irradiates a light beam emitted from a light source onto an optical recording medium to form a light spot on a track of a recording surface of the optical recording medium to read the optical recording medium while controlling the position of the light spot relative to the track of the recording surface. The optical pickup device includes a converging lens section coaxially disposed in an optical axis of the light beam to converge the light beam onto the recording surface; and a hologram lens section coaxially disposed in the optical axis of the light beam to diffract the light beam. The hologram lens section has a diffraction grating pattern being formed in such a manner that a first-order diffraction light of the light beam caused by the hologram lens section and converged on the recording surface has a first numerical aperture existing in a direction intersecting the track extending direction and a second numerical aperture existing in the track extending direction. The first numerical aperture is less than the second numerical aperture. The hologram lens section has a diffraction grating pattern including a parallel-line diffraction grating pattern and two semicircular diffraction grating patterns.

This application is a divisional of U.S. Application No. 09/005,808,filed Jan. 12, 1998 now U.S. Pat. No. 6,055,076.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup device or opticalhead device and particularly to an optical system of an optical pickupdevice in an optical recording and reproducing apparatus.

2. Description of the Related Art

There have been known as optical information recording mediums, variouskinds of optical discs, called Laser Disc (LD), Compact Disc (CD) andDigital Video Disc (DVD). Such optical discs are standardizedrespectively in different specifications such as a base thickness of thesubstrate and the like. An optical disc having a multi-layered structureprovided in a DVD specification comprises many laminated recordinglayers having effective thicknesses different from each other. Moreover,the most suitable numerical aperture (NA) of a reading objective lens isdifferent among the different optical discs.

For example, a compatible player capable of reading out recordinginformation from both the CD and DVD is required in the market. Thereare the following differences between optical systems for reading outthe recording information on the CD and the DVD.

(1) The difference of numerical apertures NA: the numerical aperture forthe CD is 0.37 and for the DVD the numerical aperture is 0.6.

(2) The difference of base thicknesses from a recording surface(reflection surface) to an outer surface of the optical disc: the basethickness for the CD is 1.2 mm and for the DVD the base thickness is 0.6mm.

Consequently, these differences must be canceled to achieve the opticalpickup device for the CD/DVD compatible player.

To achieve such compatible players, it is often thought that anobjective lens having two focal points should advantageously beemployed.

Specifically, an objective lens having a single focal point has anumerical aperture most suitable for one optical disc, the numericalaperture being improper for the other optical disc. For the otheroptical disc, such objective lens produces an aberration such as aspherical aberration.

As an example, a two-focal-point pickup device using a hologram lens hasbeen disclosed in Japanese Patent Application Kokai NO. 7-98431/1995.This two-focal-point pickup device includes a complex objective lenswhich is composed of a convex objective lens 1 and a hologram lens 2 forreading the different optical discs, as shown in FIGS. 1A and 1B. In thehologram 2, a plurality of diffraction grooves 3 are formed over theregion corresponding to the numerical aperture for the CD. A light beamis diffracted by these diffraction grooves 3, so that it is mainlydivided into a zero-order diffraction light and a first-orderdiffraction light. Thus, respective incident angles of light onto theobjective lens 1 are made different, whereby each focal point is formedon corresponding one of recording surfaces of the optical discs. At thistime, the light beam is allowed to transmit as it is through the regionwhere no diffraction groove is formed, without being diffracted, andthis transmitted light is collected by the objective lens 1 togetherwith the zero-order diffraction light. As a result, there will bedifferent in the numerical aperture between the transmitted light andthe zero-order diffraction light, and the first-order diffraction light.As shown in FIG. 1A, only the first-order diffraction light diffractedby the diffraction grooves 3 is employed for reading the CD 4 of thesmall numerical aperture. As shown in FIG. 1B, the transmitted light andthe zero-order diffraction are employed for reading the DVD 5 of thelarge numerical aperture. As seen from these figures, the hologram lensfunctions as a concave lens for the first-order diffraction light beamin this complex objective lens for reading the optical disc. This isbecause a plurality of diffraction grooves 3 or ring-shaped concave andconvex portions are evenly formed on a transparent flat plate over theregion corresponding to the numerical aperture for the CD.

On the other hand, the semiconductor laser emitting a divergent lightbeam in the CD/DVD compatible player is generally used as a light sourcein the optical pickup device. The far field pattern of the light beamhas an elliptic cross sectional intensity having a major longitudinalaxis extending in the direction perpendicular to the junction interfaceof the semiconductor laser while radiating light with a radiating angleθ in relation of θ⊥>θ∥. In other words, the light beam from thesemiconductor laser has a width of the vertical traversal mode smallerthan the horizontal traversal mode. In addition, the signal reproductionfrom DVD has a tendency of the increase of cross-talk caused by theadjacent tracks in the reproduced signal in comparison with theconventional CD, since the track-pitch of the recording pits of the DVDis narrower than that of the CD. In the view of this tendency, thesemiconductor laser for DVD is generally disposed in the optical pickupdevice in such a manner that its horizontal traversal mode correspondsto the track extending direction (also referred to as a tangentialdirection) of the optical disc, and its vertical traversal modecorresponds to the radial direction of the optical disc. Therefore, asshown in FIG. 2, the light spot 12 for DVD caused by the light beamconverged on a series of the pits 11 of the recording surface 10 of theoptical disc becomes an ellipse extending in the tangential direction.

Since only one light beam emitted from a single semiconductor laser isconverged by the common complex objective lens to be commonly used forthe reproduction of CD and DVD in the two-focal-point pickup device, alight spot 13 for reading CD in the first-order diffraction light has anelliptic shape extending in the tangential direction as well as a lightspot 12 for reading DVD as shown in FIGS. 2 and 3.

In order to precisely reproduce data from the DVD of a high recordingdensity in the CD/DVD compatible player, it is necessary to employ thesemiconductor laser emitting light with a short wavelength from 630 to680 nm less than the 780 nm wavelength of the conventional. For this,the numerical aperture of the objective lens for reading the CD used inthe CD/DVD compatible player will be set a value rather smaller thanthat of the conventional CD player. In fact, the numerical aperture forCD is set a little value larger than a numerical aperture decided inproportion to the used wavelength in the two-focal-point pickup deviceto avoid the reduction of light intensity depending on the first-orderdiffraction light and the small numerical aperture. Therefore, a lightspot 13 is formed on the recording surface 10 of the CD by thetwo-focal-point pickup device in such a manner that the diameter of beamwest of the formed light beam 13 is reduced less than the conventionallight spot size used for reading the CD as shown in FIG. 3.

In this case, if the first-order diffraction light is converged as thelight spot 13 on a long pit 11 of a series of pits as corresponding tobinary digit data “1001110” formed on the CD as shown in FIG. 4, then aglitch or bump A occasionally occurs in the signal intensity curve ofthe output from the optical pickup device, resulting in a problem that agood signal is not obtained.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problem mentionedabove, and its object is to provide a two-focal-point pickup devicecapable of suppressing the generation of glitch or bump in reproducedsignal during the playback of CD.

According to the invention, an optical pickup device irradiating a lightbeam emitted from a light source onto an optical recording medium toform a light spot on a track of a recording surface of the opticalrecording medium to read the optical recording medium while controllingthe position of the light spot relative to the track of the recordingsurface, comprises;

a converging lens section coaxially disposed in an optical axis of thelight beam to converge the light beam onto the recording surface; and

a hologram lens section coaxially disposed in the optical axis of thelight beam to diffract the light beam,

wherein said hologram lens section has a diffraction grating patternbeing formed in such a manner that a first-order diffraction light ofthe light beam caused by said hologram lens section and converged onsaid recording surface has a first numerical aperture existing in adirection intersecting the track extending direction and a secondnumerical aperture existing in the track extending direction wherein thefirst numerical aperture is less than the second numerical aperture.

The above object is accomplished by the optical pickup device abovementioned in which, when CD is played, the numerical aperture NA in theradial direction of the CD decreases so as to form a light spot with ananisotropic light intensity distribution for expanding the intensityaround the pit in the radial direction. Therefore, the inventionprevents the generation of glitch or bump in reproduced signal duringthe playback of CD, without any increase of the number of optical partsin the light pickup device.

The hologram lens section in the light pickup device for reading theoptical recording medium has an anisotropic diffraction grating patterndifferent in the radial and tangential directions selected from anelliptic pattern, an oval pattern, a double side cut circular patternand the like to expand the light intensity distribution of the lightspot in the radial direction of the optical disc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B a re schematic constructional diagrams showingtwo-focal-point complex objective lens for reading different opticalrecording mediums respectively;

FIG. 2 is an enlarged plan view showing the recording surface of a DVD;

FIG. 3 is an enlarged plan view showing the recording surface of a CD;

FIG. 4 is a diagram showing the relationship between the pit data of theCD and the intensity curve of signal read by the conventionaltwo-focal-point optical pickup device;

FIG. 5 is a schematic constructional diagram showing an optical pickupdevice of an embodiment comprising a two-focal-point complex objectivelens for reading an optical recording medium;

FIG. 6 is a front view seen from the optical axis of a hologram lenssection in the two-focal-point complex objective lens of an embodimentfor reading an optical recording medium;

FIG. 7 is a cross sectional view taken off along the line BB appearingin FIG. 6;

FIG. 8 is an enlarged plan view showing the recording surface of a CDwith a light spot formed by the optical pickup device of the invention;

FIGS. 9A and 9B are schematic constructional diagrams showing atwo-focal-point complex objective lens for reading a CD which is seenfrom the radial and tangential directions of the CD respectively;

FIG. 10 is a front view seen from the optical axis of a hologram lenssection in the two-focal-point complex objective lens of anotherembodiment for reading an optical recording medium; and

FIG. 11 is a front view seen from the optical axis of a hologram lenssection in the two-focal-point complex objective lens of anotherembodiment for reading an optical recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described below withreference to the accompanying drawings.

FIG. 5 schematically shows an optical pickup device of an embodimentaccording to the invention comprising a two-focal-point complexobjective lens for reading an optical recording medium or LD, CD, DVD,CD-R and the like. In a body of the optical pickup device, asemiconductor laser 21 of a light source, a half mirror 22, a collimatorlens 23 for converting a light beam to a parallel ray, a complexobjective lens 24 for converging the light beam onto an optical disc 30to form a light spot to read the optical recording medium, and aphotodetector 25 for receiving light reflected from the light spot areprovided. The complex objective lens 24 comprises a converging lenssection 24 a of a convex lens for converging the incident light beam onthe optical disc, and a hologram lens section 24 b of a diffractiongrating pattern formed as a Fresnel lens on a transparent flat platewhich functions as a concave lens for a first-order diffraction light.The diffraction grating pattern of the hologram lens is a plurality ofdiffraction grooves of ring-shaped concave and convex portions which areconcentrically formed on the transparent flat plate over the regioncorresponding to a specific numerical aperture for the CD specificationand arranged coaxially with the optical axis of the converging lens. Theconverging lens section 24 a and the hologram lens section 24 b arearranged to a cylindrical holder so as to be away from each other at apredetermined distance and parallel to each other.

In addition, the body of the optical pickup device accommodates anobjective-lens driving mechanism 26 including a tracking actuator and afocusing actuator. The focusing actuator allows the complex objectivelens 24 to move in the direction perpendicular to the informationrecording surface of the optical disc 30 for reading it. The trackingactuator allows the complex objective lens 24 to move in the radialdirection of the optical disc 30. Furthermore, a receiving optics system27 is disposed at the upstream of the photodetector 25 to perform aconvergence or astigmatism generation for the reflected light.

The semiconductor laser emits a laser beam to the half mirror 22 whichreflect it to the collimator lens 23. The collimator lens 23 convertsthe divergent laser beam to a parallel laser beam and pass it to thecomplex objective lens 24. The hologram lens section 24 b mainly dividesthe light beam into a zero-order diffraction light and a first-orderdiffraction light through its diffracting function. These diffractedlight beams are converged toward the optical disc 30 by the converginglens 24 a, so that a light spot is formed on a pit train of theinformation recording surface of the optical disc 30. During thereproduction of DVD, the zero-order diffraction light diffracted by thehologram lens 24 b transmits through the hologram lens together with theambient transmission light, like passing through a parallel flat glass,and is converged onto the optical disc at a numerical aperture NA of 0.6by the setting of the distance between the objective lens and theoptical disc, thereby forming a small light spot as shown in FIG. 2, soas to suppers the influence of cross-talk during the playback of theDVD.

When a CD is reproduced, by using the first-order diffraction lightdiffracted by the hologram lens 24 b, the hologram lens 24 b functionsas a concave toric lens to provide an astigmatism to the first-orderdiffraction light, so that the complex objective lens 24 forms a lightspot 13 a expanded in the radius direction on the optical disc 30.

Almost of the reflection light from the light spot of the informationrecording surface 10 of the optical disc 30 enters the complex objectivelens 24 and then passes through the collimator lens 23, the half mirror22 and the receiving optics system 27 onto a light receiving portion ofthe photodetector 25.

When an astigmatism generator and a four light-receiving-surfaceelements are used for the receiving optics system 27 and thephotodetector 25 respectively, a servo-control for the complex objectivelens 24 is performed as follows:

When a spot image is formed near the center of the fourlight-receiving-surface elements by the reflected light, thephotodetector supplies an electric signal to a demodulating circuit 32 aand an error detecting circuit 32 b in accordance with portions of thespot image formed on the four light-receiving-surface elements. Thedemodulating circuit 32 a reproduces a recorded signal on the basis ofthe electric signal. The error detecting circuit 32 b generates afocusing error signal, a tracking error signal, another servo signal,and the like on the basis of the electric signal to supply them to thecorresponding actuators through an actuator driving circuit 33. Theactuator driving circuit 33 drives the complex objective lens 24 and thelike actuators under the servo-control in accordance with the drivingsignals.

In this way, the optical pickup device having the complex objective lensreads the optical disc by forming the light spot on the recordingsurface of the optical disc while controlling the position of the lightspot relative to a track of the recording surface.

Next, the diffracting function of the hologram lens section 24 b will bedescribed. Namely, there is described in detail that the hologram lenssection has a diffraction grating pattern being formed in such a mannerthat a first-order diffraction light of the light beam caused by thehologram lens section and converged on the recording surface has a firstnumerical aperture existing in a direction intersecting the trackextending direction and a second numerical aperture existing in thetrack extending direction wherein the first numerical aperture is lessthan the second numerical aperture. FIG. 6 and FIG. 7 are a front viewseen from the optical axis of the hologram lens section and a crosssectional view thereof respectively. As seen from FIG. 6, the hologramlens section 24 b comprises an ellipsoidal diffraction grating pattern241 consisting of a plurality of ellipsoidal grooves concentricallyformed on one side of a transparent flat plate. The center of symmetryin the ellipsoidal diffraction grating pattern is coaxially placed atthe optical axis of the converging lens section. the hologram lenssection 24 b also comprises a transparent flat portion 242 sandwichingthe ellipsoidal diffraction grating pattern without any ellipsoidaldiffraction grating pattern. As seen from FIG. 7, the cross-section ofthe ellipsoidal diffraction grating pattern 241 has a saw blade shape.The ellipsoidal diffraction grating pattern 241 functions as a concavetoric lens like e a concave micro Fresnel lens. In the transparent flatportion 242, the width in the radial direction W₁ is greater than thewidth in the tangential direction W₂ with respect to a traversecross-section of the transmitting light beam (0=<W₂<W₁), so that thediffraction grating pattern 241 is arranged at the optical axis of thelight beam the portion of which passes through the transparent flatportion 242.

As shown in FIG. 8, by using the hologram lens section 24 b includingthe ellipsoidal diffraction grating pattern in the optical pickupdevice, the light spot 13 a is formed on the recording surface 10 of theoptical disc so as to be expanded in light intensity about the pits 11in the radial direction. In other words, the hologram lens section 24 bof the complex objective lens 24 a functions as a concave toric lenshaving a curved surface formed in such a manner that a radius ofcurvature in a meridional plane is different from that of the sagittalplane and sequentially increases through from the meridional radius ofcurvature to the sagittal's. The hologram lens section 24 b is set insuch a manner that the meridional plane corresponds to the radialdirection of the optical disc and the sagittal plane corresponds to thetangential direction of the optical disc. Consequently, by thecombination of the hologram lens section 24 b and the converging lenssection 24 a, i.e., the complex objective lens 24 a, a numericalaperture NA_(TAN) in the tangential direction plane on the recordingsurface 10 of the optical disc as shown in FIG. 9A is grater than thenumerical aperture NA_(RAD). in the radial direction plane as shown inFIG. 9B.

Example with the Numerical Apertures NA_(RAD) and NA_(TAN) Calculated inthe Complex Objective Lens

It is first considered that a factor of σ specifies a light intensitydistribution over the entrance pupil of the complex objective lens inthe system for a DVD. Assuming that, when an incident laser beam withGaussian distribution enters the pupil of the objective lens, thecentral light intensity is equal to I₀ (≡1), and a light intensity overthe pupil is Im.

In addition, Im/I₀=Im=exp(−σ) represents a light intensity at the rim.

The Gaussian distribution ψ(γ)=exp(−2γ²) is ψ(γ)=0.5 at a lightintensity 0.5 (half value). Thus, γ₀=[−(ln0.5)/2]^(½) is given. Aradiation angle θ₀ defines a half width at half maximum of the lightbeam irradiated from the semiconductor laser.

The normalized pupil radius γ above may be written as NA=sin θ∝γ withrespect to a radiation angle θ of the light beam.

Now, a light intensity at an optional radiation angle θ is consideredunder the conditions NA₀=sin θ₀∝γ₀, and θ₀ is a half width at halfmaximum light beam. The factor of σ specifying a light intensitydistribution is given as follows:

NA₀:γ₀=NA:γ

γ=γ₀×NA/NA₀

∴σ=2γ²

 =2γ²×(NA/NA₀)²

=2×[−ln(0.5)/2]×(NA/NA₀)²

=−ln0.5×(NA/NA₀)²

A radiation angle of the light beam from the semiconductor laser isgenerally as follows:

θ_(RAD)≡θ∥(parallel to the junction plane)=4⁰

θ_(TAN)≡θ⊥(vertical to the junction plane)=15.5⁰

at the half width at half maximum in the far field pattern.

Whilst, assuming under the conditions of a numerical aperture on theoptical disc side NA_(obj)=0.6 for DVD, an objective lens with a focallength on the optical disc side f′_(obj)=3.3 and a focal length on thesemiconductor laser side f′_(LD)=19.09 is used, then a numericalaperture on the semiconductor laser side NA_(LD) is given by

NA_(LD)=NA_(obj)×f′_(obj)/f′_(LD)

 =0.1037

At this time, a factor of σ specifying light intensity distribution overthe entrance pupil of the objective lens in the tangential direction isobtained by

σ=−ln0.5×(0.1037/sin θ₀)²

=1.532≡σ_(TAN)

Similarly, a factor of σ_(RAD) in the radial direction is calculated inthe same manner described above, resulting in

σ_(RAD=)0.104.

Therefore, the light intensities Im_(TAN) and Im_(RAD) are calculated asfollows:

Im_(TAN)=exp(−σ_(TAN))=0.216

Im_(RAD)=exp(−σ_(RAD))=0.901

As a result, an anisotropic light intensity distribution occur in thetangential and the radial directions.

When the laser beam having such an anisotropic light intensitydistribution enters the objective lens, the light intensity distributionof laser beam converged on the optical disc is narrow in the radialdirection and wide in the tangential direction as shown in FIG. 2.

Next, it is considered that a factor of σ specifies a light intensitydistribution over the entrance pupil of the objective lens in the systemfor CD. Assuming that an incident laser beam with the same distributionis used during the playback of a CD under the same conditions as the DVDsystem described above, excepting NA=0.375 for CD, then factors of σvary as follows:

σ_(TAN)=0.599

σ_(RAD)=0.041

Therefore, the light intensities Im_(TAN) and Im_(RAD) for the CD arecalculated as follows:

Im_(TAN)=exp(−0.599)=0.550

Im_(RAD)=exp(−0.041)=0.960

In this case, the light intensity distribution of laser beam convergedon the optical disc is a little wider in the tangential direction thanin the radial direction.

When CD is playbacked by a laser beam having such an light intensitydistribution the pits, the light intensity irradiated around the longand wide pit formed on the CD is insufficient, so that, as shown in FIG.4, a glitch or bump A occurs in the signal intensity curve of the outputfrom the optical pickup device.

Thus, the inventors have devised to make the width of the light spot inthe radial direction larger than that of in the tangential direction bymeans of setting the groove pitch of an ellipsoidal diffraction gratingpattern in the radial direction of the hologram lens section larger thanthat of in the tangential direction. For example, the inventors havemade a complex optical device with an ellipsoidal diffraction gratingpattern is fabricated in such a manner that a first-order diffractionlight of the light beam caused by the diffraction grating pattern andconverged on the CD has a first numerical aperture NA_(RAD)=0.25existing in a direction intersecting the track extending direction(i.e., a radial direction) and a second numerical apertureNA_(TAN)=0.375 existing in the track extending direction. Namely, thediffraction grating pattern is anisotropic and different in the radialand tangential directions so that the first numerical aperture on theoptical disc side is less than the second numerical aperture.

Then, the factors of σ and light intensities Im_(TAN) and Im_(RAD) forthe CD are calculated as follows:

σ_(TAN)=0.599

σ_(RAD)=0.018

Im_(TAN)=0.550

Im_(RAD)=0.982

By using the hologram lens section including the ellipsoidal diffractiongrating pattern in the optical pickup device, the light spot 13 a isformed on the recording surface 10 of the optical disc, as shown in FIG.8, so as to be expanded in light intensity about the pits 11 in theradial direction. As a result, the sufficient irradiated light intensityon the light spot of the optical disc has canceled the generation ofglitch or bump in reproduced signal during the playback of CD.

Example of Other Hologram Lens Sections

In addition to the above example in which the ellipsoidal diffractiongrating pattern consisting of a plurality of elliptic diffractiongrooves anisotropically formed on the transparent flat plate employedfor the hologram lens section 24 b, as shown FIG. 10, an oval pattern241 a may be used for the diffraction grating pattern as beinganisotropic in the radial and tangential direction in such a manner thatthe longitudinal axis of the oval pattern is aligned with the tangentialdirection of the optical disc. Furthermore, as shown in FIG. 11, adouble side cut circular pattern 241 b may be also utilized, which isformed by eliminating two symmetrical portion with the optical axis froma circular diffraction grating pattern functioning as a concave microFresnel lens, in such a manner that the longitudinal axis of the ovalpattern is aligned with the tangential direction of the optical disc.These both example are able to expand the light intensity of the lightspot about the pit of the optical disc in the radial direction. Thevariety of the diffraction grating pattern of the hologram lens sectionmay be adopted to adjust the light intensity distribution in the lightspot for the complex objective lens for reading an optical recordingmedium.

The hologram lens may be formed of an optical glass or plastics such aspolymetylmethacrylate (PMMA). The replica of the hologram lens may beformed thorough an injection-molding method, a photo-polymer method byusing a die with the diffraction grating pattern formed by a superprecise lathing machine such as a diamond turning machine.

Moreover, other than to use individual converging lens and the hologramlens arranged to a cylindrical holder so as to be away from each otherat a predetermined distance in the above example, there is employed asingle complex objective lens comprises a refracting convex surface onone side surface and a hologram lens of the specific diffraction gratingpattern on the other side, as a monolithic piece.

Furthermore, the optical pickup device according to the invention canconverge the light beams, at the same time, onto two-layered recordingsurfaces in the optical disc included in the DVD specificationrespectively.

In this way, the optical pickup device according to the inventioncomprises; a hologram lens section having an elliptic diffractiongrating disposed on the incident side of the laser beam and at least twotransparent flat portions symmetrically sandwiching the ellipticdiffraction grating with respect to the light beam; and a converginglens disposed on the irradiating side of the laser beam. The hologramlens of elliptic diffraction grating and the converging lens arecoaxially arranged with the incident laser light coinciding with thenormal lines of the main surfaces thereof. The area of the ellipticdiffraction grating of the hologram lens section is smaller than an areaof traverse cross-section of the incident laser beam to allow theportion of the incident laser beam to pass the two transparent flatportions.

What is claimed is:
 1. An optical pickup device for irradiating a lightbeam emitted from a light source onto an optical recording medium toform a light spot on a track of a recording surface of the opticalrecording medium to read the optical recording medium while controllingthe position of the light spot relative to the track of the recordingsurface, comprises: a converging lens section coaxially disposed in anoptical axis of the light beam to converge the light beam onto therecording surface; and a hologram lens section coaxially disposed in theoptical axis of the light beam to diffract the light beam, wherein saidhologram lens section has a diffraction grating pattern being formed insuch a manner that a first-order diffraction light of the light beamcaused by said hologram lens section and converged on said recordingsurface has a first numerical aperture existing in a directionintersecting a track extending direction and a second numerical apertureexisting in the track extending directions, wherein the first numericalaperture is less than the second numerical aperture, wherein saiddiffraction grating pattern is an oval diffraction grating patternconsisting of a longitudinal parallel-line diffraction grating patternand two semicircular diffraction grating patterns connected to both endsof the parallel-line diffraction grating pattern which are symmetricallyformed with respect to the optical axis of the light beam, and whereinsaid oval diffraction grating pattern is symmetrically formed withrespect to the optical axis of the light beam so as to have a longestwidth corresponding to the track extending direction which is greaterthan a shortest width of the pattern corresponding to the directionintersecting the track extending direction within a traversecross-section of transmitting light beam.
 2. An optical pickup deviceaccording to claim 1, wherein said converging lens section and saidhologram lens section are integrally formed as a single complexobjective lens which comprises a refracting convex surface on one sidesurface and a hologram lens having said oval diffraction grating patternon another side.